1) Field of the Invention
This invention relates to a device and method for controlling an air volume during an idle operation of an internal combustion engine such that an intake-air volume of the internal combustion engine can be adjusted to stabilize an engine speed of the internal combustion engine during the idle operation.
2) Description of the Related Art
Under conventional idle speed control of an internal combustion engine, which may hereinafter be called an “engine”, of a vehicle or the like to stabilize an engine speed during an idle operation that the internal combustion engine is idling under no-load conditions (in other words, under an internal load alone), a throttle valve or bypass valve (for example, an ISC valve) is operated to adjust an intake-air volume of the internal combustion engine. Upon conducting the idle speed control, commonly employed is PID control which makes combined use of a P correction proportionate to differences ΔNe in engine speed, a D correction proportionate to change rates dNe in engine speed and an I correction proportionate to an integral of the differences ΔNe. This PID control calculates a throttle opening correction amount by using the following basic equation.Throttle opening correction amount=Kp×ΔNe+Kd×dNe+Ki×Σ(ΔNe)where the proportional gain Kp, differential gain Kd and integral gain Ki are tuned based on a real engine.
It is, however, difficult to obtain optimal values for the individual gains Kp, Kd and Ki because they are generally determined as a result of trial and error upon development of the internal combustion engine. Moreover, it is not clear how these gains Kp, Kd and Ki should be altered when the load condition and atmosphere conditions change. Even if one tries to effect gain change-over or gain map replacements, it is difficult to adequately effect these gain change-over or gain map replacements. These problems still remain unsolved in stabilizing an idle speed.
As will be indicated by the following equation, for example, a technique has been developed in recent years to determine a throttle opening correction amount on a basis of an output torque of an internal combustion engine.Throttle opening correction amount=f(torque correction amount)where f: function map, andtorque correction amount: Kp×ΔNe+Kd×dNe+Ki×Σ(ΔNe)
Even in the above-described technique, however, no improvements have been achieved in the setting of the individual gains Kp, Kd and Ki, so that it is still difficult to adequately set the individual gains Kp, Kd and Ki.
With the foregoing in view, a further technique has been developed (for example, JP 7-197828 A). According to this technique, a target output torque is estimated by detecting an external load applied on an internal combustion engine and then reading an output torque, which is required to drive the external load, from a map in which output torques are stored corresponding to engine speeds and throttle openings. Based on the target output torque, a target throttle opening is again estimated from the above-described map.
However, the technique such as that disclosed in JP 7-197828 A estimates a target throttle opening on the basis of a map so that an accurate target throttle opening can be hardly estimated when the load conditions and atmosphere conditions change, although this technique is free of the difficulty in setting a gain that has remained as an unsolved problem to date.